Retrieval of stratospheric ozone profiles from MIPAS/ENVISAT limb emission spectra: a sensitivity study N. Glatthor1, T. von Clarmann1, H. Fischer1, B. Funke2, S. Gil-López2, U. Grabowski1, M. Höpfner1, S. Kellmann1, A. Linden1, M. López-Puertas2, G. Mengistu Tsidu1,*, M. Milz1, T. Steck1, G. P. Stiller1, and D.-Y. Wang1,** 1Forschungszentrum Karlsruhe, Institut für Meteorologie und Klimaforschung, Karlsruhe, Germany 2Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain *now at: Addis Ababa University, Department of Physics, Addis Ababa, Ethiopia **now at: University of New Brunswick, Physics Department, Fredericton, NB E3B 5A3, Canada
Abstract. We report on the dependence of ozone volume mixing ratio profiles, retrieved from
limb emission infrared spectra of the Michelson Interferometer for Passive Atmospheric
Sounding (MIPAS), on different retrieval setups such as the treatment of the background
continuum, cloud filtering, spectral regions used for analysis and a series of further
more technical parameter choices. The purpose of this investigation is to better understand
the error sources of the ozone retrieval, to optimize the current retrieval setup and to
document changes in the data versions.
It was shown that the cloud clearing technique used so far (cloud index 1.8) does
not reliably exclude all cloud-contaminated spectra from analysis. Through analysis of
spectra calculated for cloudy atmospheres we found that the cloud index should be increased
to a value of 3.0 or higher. Further, it was found that assignment of a common background
continuum to adjacent microwindows within 5 cm−1 is advantageous, because it sufficiently
represents the continuum emission by aerosols, clouds and gases as reported in the literature,
and is computationally more efficient.
For ozone retrieval we use ozone lines from MIPAS band A (685–970 cm−1)
and band AB (1020–1170 cm−1) as well. Therefore we checked ozone retrievals
with lines from bands A or AB only for a systematic difference. Such a
difference was indeed found and could, to a major part, be attributed to the
spectroscopic data used in these two bands, and to a minor part to neglection of modelling
of non-local thermodynamic (non-LTE) emissions. Another potential explanation, a bias in
the radiance calibration of level-1B spectra of bands A and AB, could largely be ruled out
by correlation analysis and inspection of broadband spectra. Further upgrades
in the ozone retrieval consist of application
of an all-zero a-priori profile and a weaker regularization. Finally, the ozone
distribution obtained with the new retrieval setup (data versions V3o_O3_7) was
compared to the data version used before (V2_O3_2). Differences are smaller
than $\pm$0.4 ppmv in the altitude region 15–50 km. Further, differences to ozone measured
by the HALogen Occultation Experiment (HALOE) on the Upper Atmospheric Research Satellite
(UARS) are partly reduced with the new MIPAS data version.
Citation: Glatthor, N., von Clarmann, T., Fischer, H., Funke, B., Gil-López, S., Grabowski, U., Höpfner, M., Kellmann, S., Linden, A., López-Puertas, M., Mengistu Tsidu, G., Milz, M., Steck, T., Stiller, G. P., and Wang, D.-Y.: Retrieval of stratospheric ozone profiles from MIPAS/ENVISAT limb emission spectra: a sensitivity study, Atmos. Chem. Phys., 6, 2767-2781, doi:10.5194/acp-6-2767-2006, 2006.